In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, one parameter in providing good performance and capacity for a given communications protocol in a communications network is packet data latency. Latency measurements can be performed in all stages of the communications network, for example when verifying a new software release or system component, and/or when deploying the communications network and when the communications network is in commercial operation.
Shorter latency than previous generations of 3GPP radio access technologies was one performance metric that guided the design of Long Term Evolution (LTE). LTE is also now recognized by the end-users to be a system that provides faster access to internet and lower packet latencies than previous generations of mobile radio technologies.
Packet latency is also a parameter that indirectly influences the throughput of the communications network. Traffic using the Hypertext Transfer Protocol (HTTP) and/or the Transmission Control Protocol (TCP) is currently one of the dominating application and transport layer protocol suite used on the Internet. The typical size of HTTP based transactions over the Internet is in the range of a few 10's of Kilo byte up to 1 Mega byte. In this size range, the TCP slow start period is a significant part of the total transport period of the packet stream. During TCP slow start the performance is packet latency limited. Hence, improved packet latency can potentially improve the average throughput, at least for this type of TCP based data transactions.
Radio resource efficiency could also be positively impacted by packet latency reductions. Lower packet data latency could increase the number of transmissions possible within a certain delay bound; hence higher Block Error Rate (BLER) targets could be used for the data transmissions freeing up radio resources potentially improving the capacity of the system.
Data on a transport channel between communicating devices in the communications network is transmitted in units known as Transport Blocks (TB), each of which corresponds to a Medium Access Control (MAC) layer Protocol Data Unit (PDU). In LTE release 10, within each transmission time interval (TTI), corresponding to one subframe duration of 1 ms, up to two TBs of dynamic size are delivered from MAC layer to the physical (PHY) layer and transmitted over the radio interface. The number of TBs transmitted within a TTI depends on configuration of multi-antenna transmission schemes. Associated with each TB is a transport format, specifying how the TB is to be transmitted. The transport format includes information about the Transport Block Size (TBS), the Modulation- and Coding Scheme (MCS), and the antenna mapping.
In order to reduce the impacts of packet latency it has therefore been proposed to shorten the subframe length. The existing TBS tables, as given in 3GPP TS 36.213, Section 7.1.7.2., are defined for a fixed length TTI of 1 ms. For a given number of assigned resource blocks, the number of available resource elements for data transmission within a shortened subframe can be significantly less as compared to the case of the legacy 1 ms TTI. Thus, the existing TBS tables cannot be used for supporting shortened subframes.
Hence, there is a need to find TBS values that are suitable for short subframes.